Magneto-optic (MO) disk drives are now being used as rewritable, high capacity storage devices. Such drives utilize the fact that a laser beam reflected by a magnetic layer is deflected in one of two opposite directions depending upon the polarity of the layer at the point of reflection. Information is written onto such a layer by heating a portion of the layer, which corresponds to one bit of information, beyond the Curie point of the layer material and then allowing the material to cool under the influence of a magnetic field, so that the desired portion of the layer assumes the orientation of the magnetic field. One polarity corresponds to "0" and the opposite polarity corresponds to "1," so that information is written in binary code. The typical magnetic layer is a Tb--Fe--Co amorphous film.
However, one great disadvantage of conventional MO disk drives is the inability to directly overwrite existing information, which results in slower write times. This problem is largely due to the fact that the magnetic field is typically applied by a square bar magnet polarized in a direction perpendicular to its axis. To reverse the polarity, the magnet must be rotated 180 degrees; this typically takes approximately 12 milliseconds. While laser pulse frequencies of 5 Mhz are not uncommon, the time required to rotate the magnet places a limit on writing speed that is too slow for practical direct overwriting.
Thus, overwriting is now normally done by first erasing an entire track by heating it and allowing it to cool under a magnetic field which polarizes all of the bits in one direction, and then rotating the magnet and pulsing the laser to heat only those bits which should be of the opposite polarity.
For this reason, a fast switching rate of the magnetic field is required for direct overwriting. One such proposal is described by Yamagami et al., in "High Density Magneto-Optical Recording with Magnetic Field Modulation and Pulsed Laser Irradiation," Conference Digest, Topical Meeting on Optical Data Storage, 1990, pp 168-171.
In this article, the authors describe the use of a thin film magnetic head for direct overwriting. The head is formed by applying a thin film layer on a Ni-Zn ferrite board. Such a head is also described in an article by Watanabe et al., in Proc. Optical Memory Symposium, Tokyo, 24988, p. 47.
The wavelength of the laser used by Yamagami et al., was 750 nm and the N.A. of the objective lens was 0.53. This results in a spot approximately 1 micron in diameter. As lasers with shorter wavelengths become available, this spot size will decrease.
As noted by Yamagami et al., a magnetic head for direct overwriting must have a uniform resistance up to 20 MHz and a wide range of magnetic field intensity as much as .+-.150 Oe, which is about the minimum field strength which can alter a recording layer of present technology. FIG. 1 illustrates a thin film magnetooptic recording head 100. The thin film magnetic head 100 is comprised of a Ni--Zn ferrite substrate 10 and a continuous coil 14 of conductive material having a plurality of windings. The coil 14 is positioned on the top, planar surface 16 of the substrate 10 as viewed in the figure, and is formed by thin film techniques, for example by depositing a layer of conductive material on the planar surface 16 and, using conventional photolithographic techniques, etching the continuous coil pattern.
A current is supplied to the coil 14 and creates a magnetic field 20 which is perpendicular to the substrate surface and the current path through the coil 14. The current through the coil 14 is electrically switched to quickly alternate the direction of the magnetic field. Hence, the head can be used for direct overwriting at a more rapid rate than that which is attainable by MO drives using bar magnets. Yamagami et al. reported a burst transfer rate of 15 Mps.
However, while the magnetic thin film head is an improvement, it also has several disadvantages. First, the head must be maintained relatively close to the MO disk. The bar magnets described above are much stronger and are normally located about 0.5 mm away from the disk. Even at this distance, the magnetic field is about 250 Oe, sufficient to allow for the normal up and down wobble of the disk, which can be as much as 0.3 mm. The thin film head of Yamagami et al., on the other hand, generates a field of 150 Oe at a distance of 0.1 mm, or 100 microns. This is in part due to the fact that the head suffers from flux leakage. Thus, the magnetic head must remain within 0.1 mm of the recording layer so that the field strength at the disk is sufficient for writing. To accomplish this, the head must move with the disk. Further, such close proximity creates a danger of contact between the head to the disk.
Another disadvantage of the thin film coil head described by Yamagami et al. is that its impedance causes its alternating current resistance to increase rapidly as the frequency of the current exceeds 20 Mhz. This imposes another limit on the speed of overwriting which is attainable with such a thin film head.
Still another disadvantage of the thin film head described by Yamagami et al. is that current flows through the coil continuously, and this causes the head to get very hot, which shortens the useful life of the head.
An improvement to the the thin film head of Yamagami et al. is shown in Japanese Utility Model Application No. 01-136416, which teaches the use of two elongate coils, one superimposed on the other, in which the windings of one coil run in the clockwise direction and those in the other coil run in the counter-clockwise direction. This structure, while solving suffers from flux leakage and heat dissipation some of the problems of the single coil head, still problems.
It is therefore an object of the present invention to provide a magneto-optical recording head which provides increased disk to head spacing and thus minimizes the risk of contact between the head and the disk.
Another object of the present invention is to provide a magneto-optical recording head having means which minimizes magnetic flux leakage at the head, thereby using less current to establish a magnetic field, or, alternatively, generating a greater field at a given level of current.
A further object of the invention is to provide a magneto-optical recording head which allows more rapid switching of the magnetic field so as to achieve a fast data recording rate.
Another object of the invention is to provide a magneto-optical recording head with an improved heat dissipating capability.